lowering compression on 502?
 

Hi everyone...

I've got a gen 6 volvo penta injection 502 block which I was going to supercharge, but I've found it's high compression (8.something:1) I'm told it's pretty pricey to lower this with pistons as it'll also need different rods.

Is there another option out there to lower the compression that's (hopefully a lot) cheaper? Are there thicker headgaskets etc? I know felpro do a thicker gasket, but I'm told I need to drop compression by quite a bit, down to the low 7's?

I guess my only other option is to stay N/A but it seems a shame to not use such a beautiful blower stack.

Cheers, James

To my knowledge 8.7:1 is fine for a blower.

not sure if its the same world but depending on how much boost you run, say 8.5.1 comp, you should be fine with boost around the 4-6 psi range, i relaying this from an automotive standpoint where 8 something to 1 is pretty low comp and to add a blower would not be a problem

8:1 is what I have on my 502 blocks. Running 7 lbs of boost. Nevertheless, if your internals are not high quality and designed to handle the blowers, you are asking for trouble. You are going to break your motors.

The stock 8.75 compression is fine to be supercharged as long as you keep boost around 5# and run 93 octane fuel.

You could have some port work done to the heads to enlarge the chambers and that would reduce compression by maybe .25. I would not go with a much thicker head gasket as it will increase the quench and actually increase the risk of detonation.

That compression in the 8's is just fine. Use the .060" Cometic head gaskets and then make sure the air fuel ratios are set and kept in the 11.0 to 1 to 11.8 to 1 range under loads and make sure the fuel system is capable of delivering the correct amount of fuel and at correct pressures.

Best Regards,
Ray @ Raylar

Thanks for the help guys, that's all very interesting! My engine builder was insistant that it was too high for the blower, though I did think it didn't seem all that high.

Internals are standard Volvo Penta forged 502 pistons with an Eagle crank, standard iron heads, roller rockers etc.

I'm only looking to run 3-5psi boost to just give it a little tickle anyway, would this be ok?

Cheers, James

It will run like a raped ape at 5psi. Just keep the a/f under 12:1 and back out the full timing to around 30*.

BTW, engine builders make money building engines. New pistons = engine build. No pistons = no engine build.

It will be fine as long as you run good fuel.

Here is a link to some good info and a chart that shows static vs effective compression ratios with boost.
http://www.holley.com/data/TechServi...ech%20Info.pdf

I would leave it alone as stated above. I would not however use a .060 head gasket. If the short block is stock then the piston is about .020 in the hole. With a standard .040 gasket, you have a quench of about .060. That is just fine. With a .060 gasket, you will raise the quench to .080, which is to high. It will be more susceptable to detonation, which is already a potential problem with the cast iron heads. Just run a good gasket (read: Cometic) and let it rip. You may also seriously consider a good set of exhaust valves.
I recently did a bone stock 502 mag with an M3 Procharger. At 5000 rpm and 3.4 psi boost, it made 625 hp and 681 ft lbs at 4000 rpm. It made for a really nice little piece. Good luck. give me a shout if I can helpin any way.
Eddie

You need a new builder.......

Quick update...

My motor is actually 8.81:1, so pretty close to 9:1.
According to the PDF linked earlier, this only equates to below 12:1 effective CR at below 5 psi boost, so not much.

I'm only looking to make 600hp max, as my bravo definately won't cope past that. Is it really worth me bothering with the blower and any potential issues, or am I better off buying a set of stelling headers I can get, a decent cam and building a mild N/A motor, which would also be kinder on my drive?

Cheers, James

you are gonna have to baby that bravo if its a stock setup, i think there only good to around 400hp, with 600 its a ticking bomb, if you have the coin to at least upgrade the internals on the drive and do the blower, go for it, don't forget external steering comes into play aswell, if not i would go with the mild n/a build until you can do it right

Bravo's can handle 600hp if driven right. The Merc 600SC used to be paired with a regular Bravo, as was the 500HP and 525SC.

Get some mild port work done. It will add about 2cc of volume to the chambers and lower the static Comp ratio to about 8.7 and will also flow better. Then use a .040 Cometic head gasket. Run 4-4.5# of boost and call it good.

i stand corrected, i'm learning alot from you lately griff!

One clarification may be needed. Most vovlo penta engines use the GM Vortec style cylinder head. If this is the case the combustion chamber is very prone to detonation. If it has standard GM style heads, I agree with everything said previously. Thanks.

Volvo DP high performance engines in the big block family do not use the GM Vortec (454) style head and combustion chamber but use the GM performance BBC rectangle port standard BBC combustion chamber and are not more prone to detonation.
Iron heads under big boost with low octane fuel, lean burn or to much timing will tend to detonate no mater what the combustion chamber size and type and tend to do so quicker than an aluminum head of the same type does.

Best Regards,
Ray @ Raylar

Thanks Ray, I just bought a take-off intake and injection set-up and was expecting it to fit the GM heads but it is for the GM Vortec style. Do your aluminum heads have this bolt pattern to accept this manifold. I cant even find donor steel heads to put an engine together. Thanks.

Fixx
 

what does the intake look like?? Sounds like you have a 8.1l(496) intake then and not the 8.2 l (502) how mant ports in the lower manifold including the water ports,if theirs only 8 ports then it may be a 8.1 liter intake set up.

It's a match to the GM Vortec 454 used in the trucks. The throttle body is on the side at a 45 degree angle, the upper plenum is crazy large, externally it looks identical to the truck engine in a freinds 3500 dually. At least I got a computer an injectors to use. Its my bad for not checking it out completely. I think I'll make a mailbox out of the intake. :lolhit: